The invention relates to a workpiece mounting pallet for machining and assembly systems in which the pallet is transferred through one or more stations, and a method of palletizing workpieces for transfer through the machine.
In flexible machining and assembly systems and especially in transfer lines, an assortment of workpieces which are similar in kind is often machined by several stations which are linked by a transfer system. The stations are usually specially equipped to carry out particular machining or other operations. These operations can be assembly operations, cutting operations or other machining operations. In any case, each of the stations carries out a repetitive operation on the workpiece which is brought into the station by the transfer system and held fixed by a clamping mechanism.
The clamping mechanism in the station is not adjusted between cycles, requiring perfect functioning of the transfer line over quite a long period of time. This results in high demands, above all, on the clamping mechanism which must properly fix and lock the respective workpieces in the stations during the operation. All of the workpieces have to be positioned exactly within a close tolerance in relation to critical points, automatically and without supervision by staff. Also, particularly when it is a cutting operation that is to be carried out on corresponding surfaces to be machined, the clamping mechanism has to receive and absorb considerable reaction forces which occur as a result of the machining. It is imperative that the positioning of the respective workpiece be maintained to ensure the quality of the machined workpieces.
In order to take this into account and not to have to reposition and lock in every machining station a workpiece which may be of complicated design, it is standard practice to use workpiece pallets which have clamping places provided on them and which are firmly clamped to the workpiece and cyclically transferred through all the machining stations together with the workpieces. With workpieces clamped at a precise location on the workpiece pallet, only the workpiece pallets have to be positioned, fixed and locked in the stations.
In a manner known per se, the transfer line comprises for this purpose supporting rails which are arranged on a fixed bed and on which the workpiece pallets are cyclically transferred in the advancing direction by transfer bars or other similarly operating drive means arranged between the supporting rails. For this purpose, the transfer bars which execute a lift and carry movement are periodically brought by positive or frictional connection into engagement with the workpiece pallets.
In the machining stations, fixing means, for example, locating pins are arranged in or on the supporting rails to align the respective workpiece pallet in a precisely defined position and fix it in both the advancing direction and the lateral direction in relation to the latter. The workpiece pallet is locked in this position by means of clamping jaws.
Adaptation of the transfer line to different workpieces without causing a serious disturbance in the production process can only be carried out by changing the workpiece pallets or converting these accordingly. However, a precondition for the changing of the workpiece pallets to correspond to the workpieces to be machined is that at least one specially designed workpiece pallet be held ready for each workpiece type. Owing, on the one hand, to the high accuracy requirement for the clamping of the workpieces on the workpiece pallets with long-time stability and, on the other hand, to the high reaction forces to be absorbed by the workpiece pallets, the workpiece pallets to be held in readiness are elaborate and expensive, and their cost increased by the need for their storage.
For the aforesaid reasons, various workpiece pallets have been developed to be convertible to be used for different workpiece types. Heretofore, such conversion possibilities have resulted in complicated and costly workpiece pallets.
For example, in the manufacture of clevis joint parts for motor vehicles, clevis joint parts of different dimensions have to be machined in the same transfer line, and it is often different clevis joint parts that are to pass through the machining stations immediately one after one the other in the machining sequence. The object is to be able to machine clevis joint parts in the transfer line without having to reequip the machining stations each time.
German patent 39 40 635 discloses a workpiece pallet as a workpiece carrier in production or assembly systems for the selective supporting of cylinder blocks of in-line or V-engines. For this purpose, a single clamping place is provided on the top surface of the workpiece pallet. A total of five stop rails extending parallel to one another are provided as support means on a concentric circle about an axis marked by the crankshaft bore. Two stop rails, in each respective case, which are not adjacent to one another and are separated by a stop rail located between them are allocated to one another and provided for positioning on a transfer system. The workpiece pallet can thus be placed on the transfer system in three rotated positions, turned about the horizontal axis marked by the crankshaft bore.
The workpiece pallet is specially provided for the production of cylinder blocks of V- or in-line engines. Accordingly, it has only one clamping place for two cylinder block types selectively. Owing to the arrangement of the stop rails on the above-described circle, accessibility to the workpiece is limited.
Also known in practice are workpiece pallets which have a main body with a rectangular base area and a pentagonal cross-section which can be placed on a transfer system. Adjoining the base areas, which is provided with holding and fixing devices for cooperation with the transfer system, are side surfaces which stand perpendicular on both sides of the base area and pass into inclined side surfaces. The inclined side surfaces border on a cover area which extends parallel to the base area and on which clamping places are provided for workpieces. Clamping jaws for fixing the workpieces in a machining position are arranged immediately beside the clamping places. Supporting rails and fixing bores are provided as holding and fixing devices on the base area of the workpiece pallet. Further devices for actuating the clamping jaws and for adjusting the clamping places are arranged inside the pallet.
Conversion of the workpiece pallet from one workpiece type to another, if provided at all, is costly and time consuming. Consequently, with quite a large assortment of workpieces to be machined, a corresponding supply of workpiece pallets has to be held ready. As the workpiece pallets are relatively expensive, the cost for the workpiece pallets alone and the necessary storage is considerable. A further disadvantage is that the number of clamping places arranged on the cover area is rather small.
Furthermore, so-called clamping cubes which have a right parallelepiped main body with a square main and covering area are used in practice for supporting workpieces in machine tools. The main area is provided with connection means to enable the clamping cube to be rigidly coupled to a rotatable workpiece table. Clamping means for attachment of smaller workpieces which are to be machined one after the other are provided on the side faces of the clamping cube. To selectively bring the workpieces clamped on a selected side face into the machining position, the workpiece table is turned together with the rigidly coupled clamping cube about an axis which stands perpendicular on the main area of the clamping cube.
This clamping cube is not suitable for use in transfer lines and cooperation with a transfer system.
Therefore, the object of the invention is to create a low cost workpiece pallet with which it is possible to perform machining or other operations on different workpieces in a transfer line.